1. Field of the Invention
This invention relates to high-density, electronic packaging arrangements and more particularly to a modular electronic system and stackable electronic circuit unit therefor having a unique cooling configuration which is suitable for the cooling of dual in line packages and other discrete electronic circuit packages and which permits such packages to be connected with wave soldering techniques.
2. Description of the Prior Art
The development of miniaturized solid state electronicomponents and integrated circuits together with the environmental demands of applications, such as aircraft and missile use, for example, have created a need for high-density, electronic packaging arrangements which have adequate power handling capacity and are of small size and weight. For a given volume or size of electronic packaging, an increase in the heat dissipation ability of the package will increase the power handling capacity of the package because of the constant need to provide for adequate cooling of the electronic components. Accordingly, modular electronic systems have been developed which not only reduce the physical size and volume of the package but also increase the power handling capacity of the package through the use of sophisticated cooling techniques. In these systems, the electronic circuitry is broken down into easily-replacable electronic circuit units or "modules" and a fluid coolant, such as air, for example, is forced through the modular system to dissipate the heat produced by the electronic components. The heat transferred to the fluid coolant is then dissipated in the ultimate heat sink. In addition to the foregoing requirement of good power handling capacity in a small physical size, the modular electronic system must be physically rugged and be able to withstand the high shock and vibration conditions inherent in many applications. The modular electronic system should also be easily expandible to accomodate varying numbers of the modules or circuit units which form the system.
In several prior art arrangements, the modular electronic system is cooled by a large, solid heat sink of thermally conductive material. These arrangements, however, are not thermally efficient and materially add to the size and weight of the system. The electronic circuit units of the system comprise stackable, rectangular "cards" which have one or more solid sheets of heat conductive material, such as copper or aluminum, for example, which are thermally coupled to the electronic components of the card. When the cards are stacked, the ends of the thermally conductive sheets project into a stream of fluid coolant passing through an entrance plenum disposed along one edge of the stack, so that the heat from the components passes by thermal conduction through the sheet to the ends of the sheet and from the ends of the sheets into the stream of coolant. When the electronic circuits to be mounted on the card take the form of discrete electronic circuit packages, such as dual in line packages (DIPS), for example, having specially formed and aligned leads, the solid conductive sheets must be provided with electrically insulated openings to permit the leads from the DIPS to pass through the sheets or, alternatively, the sheets themselves may be replaced by solid, thermally conductive rails. Apart from thermal inefficiency, these arrangements do not uniformly cool the cards in the stack because the ends of the solid, thermally conductive members of the cards are coupled in "series" fashion to the stream of coolant, with the result that the cards nearest the entrance point of the coolant into the entrance plenum receive maximum cooling, while the cards located farthest from the coolant entrance point receive lesser cooling because the coolant is at a higher temperature.
A so-called "hollow card" arrangement has been developed wherein solid sheets of heat conductive material are mounted on both sides of a hollow frame and the circuit components to be cooled are mounted on printed circuit boards which are laminated to the heat conductive sheets. The ends of the cards have openings which form integral entrance and exit plenums which communicate with the hollow space in the card formed by the sheets of heat conductive material so that the fluid coolant passes from one end of the card to the other. This arrangement, which is shown in U.S. Pat. No. 3,648,113 issued Mar. 7, 1972 and assigned to the assignee of the present invention, is thermally efficient and the paths for the fluid coolant through the stacked cards are effectively in parallel between the entrance and exit plenums, so that each card is uniformly cooled and receives fluid coolant at the same temperature. This arrangement, however, is principally suited to the mounting of lap soldered components, such as flat packs and the like. In order to mount DIPS and other discrete electronic circuit packages having similar lead configurations, special lead forming and mounting techniques are required together with the use of special mounting spacers as described in said U.S. Pat. No. 3,648,113. Since DIPS readily lend themselves to modern, automated, wave soldering techniques, it is apparent that a need exists for modular electronic systems and electronic circuit units therefor which will mount and cool DIPS and other discrete electronic circuit packages which are interconnected by wave soldering.